The present invention relates to a semiconductor laser device and an optical disc unit, in particular to a semiconductor laser device that can realize high output and high reliability, and an optical disc unit using the same.
Semiconductor laser devices are used in optical communication devices, optical recording devices and so on. Recently, there are increasing needs for high speed and large capacity in such devices. In order to meet the demands, research and development has been advanced for improving various characteristics of semiconductor laser devices.
Among them, a 780 nm band semiconductor laser device, which is used in an optical disc unit such as a conventional CD or CD-R/RW, is usually made of AlGaAs materials. Since demands for high-speed writing have been increasing also in the CD-R/RW, high-output semiconductor laser devices are requested in order to satisfy these demands.
As a conventional AlGaAs semiconductor laser device, there is one as shown in FIG. 10 (see, e.g., JP 11-274644). The structure of the AlGaAs semiconductor laser device will be briefly described. As shown in FIG. 10, on an n-type GaAs substrate 501, there are an n-type GaAs buffer layer 502, an n-type Al0.5Ga0.5As lower cladding layer 504, an Al0.35Ga0.65As lower guide layer 503, a multiquantum well active layer 505 composed of two Al0.12Ga0.88As well layers (each layer having a thickness of 80 Å) and three Al0.35Ga0.65As barrier layers (each layer having a thickness of 50 Å) disposed alternately, an Al0.35Ga0.65As upper guide layer 506, a p-type Al0.5Ga0.5As first upper cladding layer 507 and a p-type GaAs etching stopper layer 508 that are stacked in this order. A mesa stripe-shaped p-type Al0.5Ga0.5As second upper cladding layer 509 and an eaves-shaped p-type GaAs cap layer 510 are sequentially formed on a surface of the etching stopper layer 508. An n-type Al0.3Ga0.3As first current blocking layer 511 and an n-type GaAs second current blocking layer 512 are stacked on both sides of the second upper cladding layer 509, so that regions other than the mesa stripe portion are defined as current constriction portions. A p-type GaAs planarizing layer 513 is formed on the second current blocking layer 512, and a p-type GaAs contact layer 514 is laid on the entire surface thereof.
The semiconductor laser device has a threshold current of 35 mA and a COD (Catastrophic Optical Damage) level of about 160 mW.
However, in the semiconductor laser device that employs the AlGaAs materials, “end-face damage” caused by COD is liable to occur on laser light-emitting end faces during the high-power operation, due to influence of active Al (aluminum) atoms. As a result, such a semiconductor laser device only had a maximum optical output of about 160 mW. The end-face damage caused by COD is presumed to occur by the following mechanism. In the end faces of a resonator, because Al is easily oxidized, a surface level is formed thereby. Carriers injected into the active layer are relaxed through the level, when heat is emitted. Therefore, the temperature increases locally. The increase in the temperature reduces the bandgap of the active layer in the vicinity of the end faces. As a result, absorption of laser light in the vicinity of the end faces increases, and the number of carriers that are relaxed through the surface level increases resulting in further generation of heat. By repeating such a positive feedback, the end faces are finally melted resulting in stop of oscillation. Since Al is contained in an active region in the conventional semiconductor laser device, the end-face damage on the basis of the above principle becomes a big problem.
The present inventors have proceeded with the study on high-output semiconductor laser devices that employ InGaAsP materials that contain no Al (Al-free materials). As a result, a semiconductor laser device having a maximum optical output of up to almost 250 mW was realized, but sufficient reliability and temperature characteristics were not obtained. Inspecting this semiconductor laser device, the inventors found the possibility that carriers injected in the active region were liable to leak to the outside of the active region under high-temperature atmosphere or in high-power operation.